Metronome doesn't increase consecutive hits of a single use of a multi-hit move (say Rock Blast hits 3 times. They do roughly the same amount of damage). The only way for Rock Blast, and others like it, to benefit from Metronome is to use it repeatedly, which isn't really recommended, as the Metronome isn't a recommended item in general.

Well... if you are battling against E4, it can prove very useful since E4 has the same type of Pokémon
For example, good use of Metronome is Lucario with Shadow Claw battling Shauntal. You keep spamming Shadow Claw and you pretty easily obliterate her team